1. Technical Field
The present disclosure relates to audio devices especially utilizing an output amplifier connected to a speaker, the output amplifier capable of entering a saturation state.
2. Description of the Related Art
An audio device generally includes upstream stages, the main function of which is to process or to decode and to pre-amplify an audio signal stored in any support, for example, an internal memory, an optical disk, or any incident audio signal, and an output power amplifier, having the function of generating an analog audio signal adapted to output speakers.
FIG. 1A schematically shows the terminal portion of an audio device. An amplifier 1 receives an audio signal VIN, generated by upstream stages, and delivers an audio signal VOUT, which is an amplified image of signal VIN, to a speaker 3 of a known configuration. Amplifier 1 has a maximum output amplitude VCC determined by its supply voltage.
FIGS. 1B and 1C show examples of the variation of audio signal VOUT along time.
FIG. 1B shows a case of optimal use of amplifier 1, that is, where the amplitude of the output audio signal is maximum (equal to VCC).
FIG. 1C shows another example of the variation of audio signal VOUT along time. In this case, amplifier 1 is strongly saturated and the output signal is clipped. The characteristics of the sound signal are then altered, especially by the introduction of unwanted harmonics.
The power of output signal VOUT is equal to the surface area delimited by the curve representative of the signal and by the straight line representing the midpoint, or common-mode D.C. voltage, of amplifier 1. This surface is shown by the hatched areas of FIGS. 1B and 1C. Due to the clipping of the signal peaks, the output power increases along with the saturation. The power of an optimal output signal, such as shown in FIG. 1B (amplitude equal to VCC and no clipping), corresponds to the nominal power of amplifier 1. In case of a very strong saturation (FIG. 1C), the power of output signal VOUT may double with respect to this nominal power. Further, the signal then includes a large number of distinct harmonics that may be critical for the speaker.
Thus, a clipping in case of a saturation implies to provide a speaker 3 capable of withstanding up to twice the nominal power of the output signal. This is regrettable and is contrary to today's miniaturization aims.
An alternative solution to the clipping of output audio signal VOUT in case of a saturation is thus desired.
A protection method, currently designated as “soft-clipping” in the art, which systematically rounds off, by filtering, the audio signal peaks, enables elimination of slight saturations. A disadvantage of this method is that it decreases the nominal power of the amplifier. Further, the signal is distorted, even when the amplifier is not saturated. This protection is further inefficient against strong saturations.
Automatic gain control methods, currently designated in the art as AGC, are also used. These methods include bringing too intense peaks of the audio signal down the maximum output amplitude VCC of amplifier 1. Means for measuring the average value of output voltage VOUT are connected to means for controlling the gain of amplifier 1. Such methods strengthen the low-amplitude portions of the signal and attenuate the strong portions. A disadvantage of this type of method is the loss of data as to the amplitude of the audio signal. Further, the measurement of the average value of voltage VOUT at the output of amplifier 1 delays the gain adjustment.
A maximum saturation rate in signal VOUT capable of being withstood by speaker 3 without causing any damage is generally determined (for example by simulation). It is thus desirable to be able to accurately determine the saturation rate of output signal VOUT. Existing solutions for measuring the saturation rate, especially based on measurements of the average level of output voltage VOUT, are inaccurate and introduce delays.